1. Field
The application is related to techniques for mapping internal structures in the body of an animal or human, and more particularly to such technique for localizing a medical instrument with respect to anatomical features or the like during surgical or other medical procedures.
2. State of the Art
Various scanning apparatus and methods are known for imaging and mapping body structures, which provide target location data for surgical and other medical procedures. One group of methods, including still photography, videography, radiological x-rays, and angiography, typically produces only a two-dimensional projection of a three-dimensional object. For purposes of this application, this first group will be termed xe2x80x9ctwo-dimensionalxe2x80x9d or xe2x80x9c2Dxe2x80x9d imaging.
A second group of methods, of which computerized tomographic (CT) scanning, positron emission tomography (PET) scans, and magnetic resonance (MRI) imaging are exemplary, provides three-dimensional (abbrev. xe2x80x9c3Dxe2x80x9d herein) information about internal structures (i.e., structures not visible from the exterior of the patient). The three-dimensional information about the internal volume is reconstructed from multiple scans of a known thickness (generally about a millimeter) made along parallel planes displaced from each other by a known distance, usually of the order of millimeters. An example of such a reconstructed volume image is depicted in FIG. 1A, including the contours of a selected anatomical feature within the brain. In this application, methods of this second group will be referred to as xe2x80x9cvolumexe2x80x9d scanning or imaging.
In performing resection or other surgical manipulations, it is highly desirable to correlate the location of instruments, patient anatomical features, or other elements or structures placed in the surgical field, and generally as seen by the surgeon, with the location of internal targets or features as visualized by one of the volume scanning techniques. Such a correlation process is often termed xe2x80x9clocalizationxe2x80x9d.
A commercially available device for localization in neurosurgery is the Brown-Roberts-Wells (abbrev. BRW) localizer (U.S. Pat. Nos. 4,341,220, and 4,608,977). The BRW system includes a large ring-like structure which surrounds the patient""s head and is fixed in place. The ring establishes a 3D coordinate system with respect to the patient""s head. A separate calibration unit having an array of rod elements is fixed to the ring to surround the head during the production of volume scan and/or 2D images. The rod elements have known coordinates in the 3D coordinate system established by the ring, and produce spots in the volume scans. Other features in the volume scans can then be assigned coordinates in the 3D coordinate system established by the ring, by correlation with the known coordinates of the rod elements producing the spots.
After the images are made, the calibration unit is detached from the ring, and a guidance arc calibrated to the 3D coordinate system of the ring is attached in its place. The guidance arc provides coordinate reference information which may be uses to guide a medical instrument. The medical instrument is usually attached to the guidance arc.
The BRW system has several disadvantages. The ring is cumbersome and uncomfortable for the patient, but it must be affixed in place when the volume and/or 2D scans are made, and kept there until the medical procedure is complete. It is possible to remove the ring after the scans are made, but precise repositioning is critical to avoid error in localization. Accurate repositioning is difficult, so present practice generally is to keep the ring in place until after the surgery. When not attached to the guidance arc, the position of a medical instrument in terms of the 3D coordinate system of the ring, and therefore in respect to the features identifiable in the volume or 2D scan, is not accurately known.
U.S. Pat. No. 4,618,978 to Cosman discloses a localizer device for use with a BRW-type system, including an open box composed of connected rods, which surrounds the patient""s head and constitutes a calibration unit.
Alternatively, cranial implants of radio-opaque or MRI-opaque materials can be made. Generally, a minimum of three implants are required for establishing a three-dimensional space in volume scans. At present this method is considered very undesirable, in part because of the risk of infection or other complications of the implants.
Accordingly, a need remains for rapid, reliable, and inexpensive means for localizing a medical instrument relative to points of interest including both visible anatomical features and internal features imaged by volume and/or 2D methods. A need further remains for such means which does not require the physical attachment of a reference unit such as the BRW ring to the patient. Highly desirably, such means would be useful to track the position of a medical instrument in real time, and without requiring that the instrument be physically attached to a reference guide.
A coordinate system may be thought of as a way to assign a unique set of numerical identifiers to each point or object in a selected space. The Cartesian coordinate system is one of the best known and will be used in this paragraph by way of example. In the Cartesian coordinate system, three directions x, y, z are specified, each corresponding to one of the three dimensions of what is commonly termed 3D (three-dimensional) space (FIG. 1B). In the Cartesian system, any point can be identified by a set of three values x, y, z. The x, y and z directions can be said to establish a xe2x80x9cthree-dimensional frameworkxe2x80x9d or xe2x80x9ccoordinate frameworkxe2x80x9d in space. A selected point xe2x80x9cAxe2x80x9d can be described in terms of its values xa, ya, za; these values specify only the location of point A. A different point B will have a different set of values xb, yb, zb. Such a set of values x,y,z for any selected point is referred to herein as the xe2x80x9ccoordinatesxe2x80x9d or xe2x80x9clocational coordinatesxe2x80x9d of that point. When the position of a feature larger than a single point is being described, these terms are also understood to refer to a plurality of sets of x,y,z values. Other types of coordinate systems are known, for example spherical coordinate systems, and the terms xe2x80x9ccoordinatesxe2x80x9d and xe2x80x9clocational coordinatesxe2x80x9d should further be understood to apply to any set of values required to uniquely specify a point in space in a given coordinate system.
The term xe2x80x9cfiducialxe2x80x9d is used herein as generally understood in engineering or surveying, to describe a point or marking, or a line, which is sufficiently precisely defined to serve as a standard or basis reference for other measurements.
The invention comprises apparatus and a method for defining the location of a medical instrument relative to elements in a medical workspace including a patient""s body region, especially (but not limited to) elements seen by the surgeon. The apparatus develops a calibrated 3 dimensional framework of the workspace from a pair of 2D images made from different fixed locations, and aligns the workspace framework with a 3D scan framework defined by a volume scan. A pair of video cameras is the present preferred imaging means for obtaining the 2D image pairs. The apparatus is then operable to locate and track the position of a medical instrument during a medical procedure, with respect to features observable in either the workspace images or in the volume scan. A pictural display of such location and tracking information is provided to aid a medical practitioner performing the procedure.
In a further embodiment, the computing means is operable to automatically recognize and track the position of selected medical or surgical instruments during a procedure, from the workspace images.
The apparatus may be described as follows. Workspace imaging means are provided and positioned for producing a plurality of pairs of 2-dimensional images of a medical workspace. Each image pair comprises two such images made in effect simultaneously along respective different sightlines which intersect at an angle. Digitizing means are operably disposed for digitizing each image to produce corresponding sets of digital output signals, one set for each image.
Calibration means are removably positionable in the workspace for calibrating the workspace in terms of a three-dimensional coordinate framework. The 3D workspace framework is derived by computation from the two 2D projections of an image pair made with the calibration means positioned in the workspace. The calibration means comprises a set of at least six fiducial points connected by a frame means consisting of a frame constructed to hold the fiducial points in fixed spatial relation to each other. Although a calibration means with a set of at least six fiducial points is preferred, it is understood that the calibration means only requires a sufficient number of fiducial points to derive the 3D workspace framework. The frame need not include any means for attaching the fiducial points to a patient. The set of fiducial points has known spatial parameters which define an arbitrary Cartesian 3-dimensional coordinate system. These spatial parameters include 3D location coordinates of each of the fiducial points. Optionally but desirably, at least some of the actual distances between fiducial points should be known, to calibrate the workspace in terms of a suitable distance unit such as millimeters.
A computing means is connected to receive the digital output signals reflective of the images. The computing means also has data input means for receiving scan data from a volume scan of the patient""s body region. The scan data define a scan 3D coordinate framework and internal anatomical structures therein. The computing means is further constructed or programmed to perform the following steps: 1) establish a workspace coordinate framework in three dimensions from an image pair made with said fiducial structure positioned within the workspace; 2) determine the locational coordinates in the workspace framework of any selected point which can be identified from both images of said pair; 3) correlate the scan locational coordinates for each of three or more selected landmarks observable in the scan with the workspace locational coordinates of the same landmarks as derived from a video image pair; 4) use the correlation of the workspace coordinates and the scan coordinates of the landmarks, to derive a transformation algorithm for mapping selected other features from either the scan framework to the workspace framework, or the converse; and 5) provide display signals encoding a display reflective of one or both of the workspace images and/or a volume scan, as selected by a user. Display means are provided for displaying the images encoded by the display signals.
Optionally but highly desirably, the computing means has computer graphics capability for producing graphic icons overlaid upon the displayed images. Such icons include a cursor which the user employs to select features in the displayed images for computation of their coordinates or other operations.
A method of surgical guidance may be described as follows. First, a fiducial structure having six or more fiducial points defining two distinct, non-orthogonal planes is positioned in a medical workspace. Workspace imaging means are disposed for making pairs of two-dimensional images of the workspace in which the two member images are made along different but intersecting sightlines. A calibration image pair comprising images of the workspace with the fiducial structure is made. The fiducial structure is removed from the workspace.
A projection algorithm is applied to reconstruct a workspace 3D coordinate framework from the calibration image pair. At least one additional 3D scan framework is obtained from a corresponding volume scan of the patient""s body region. At least three landmarks identifiable in both the volume scan and the workspace image pair are selected, and the coordinates for the three landmarks are determined in both the workspace framework and the scan framework. From these determined coordinates, a process is developed for aligning the scan framework with the workspace framework, and transformation algorithms for converting coordinates from one of the frameworks to the other are computed.
A target of interest in the volume scan is identified, and its scan coordinates are determined and converted to workspace coordinates. A feature of interest in the workspace, such as a fiducial mark on a scalpel, is identified. The workspace coordinates of the fiducial mark and of the scalpel tip (whose distance from the fiducial mark is known), plus a vector describing the direction of the scalpel, are determined. Optionally but highly desirably, both the target and the scalpel including the scalpel tip position are displayed in an image of the workspace. The path of the scalpel tip is extrapolated along the vector for a distance sufficient to determine whether the tip will reach the target on this path. If not, the direction of the scalpel is adjusted and the process of localizing the tip and extrapolating its path is repeated until the extrapolated path is deemed adequate by a user, and/or until the medical procedure is complete.
The invention also includes a fiducial structure for establishing a three-dimensional coordinate framework for a photographic image pair, comprising a sufficient number of fiducial indicators arranged to a three-dimensional coordinate system and frame means for supporting and connecting the fiducial indicators, wherein the frame means consists of a frame constructed to hold the fiducial indicators in fixed relation to each other.